1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus. More particularly, the present invention relates to a photosensitive unit having a cover that covers an exposed portion of a photosensitive medium and an electrophotographic image forming apparatus including the photosensitive unit.
2. Description of the Related Art
Generally, an electrophotographic image forming apparatus forms an electrostatic latent image by scanning lights on a photosensitive medium that is charged to a predetermined potential. The electrostatic latent image is developed with a toner using a developing unit. The developed image is transferred and fused on a sheet of paper to print a mono-color image or a full-color image.
The electrophotographic image forming apparatus that prints the full-color image generally includes an exposure unit scanning beams of light corresponding to image information. The electrostatic latent image is formed on a photosensitive medium by the light scanned from the exposure unit. Four developing units respectively receive toners of four colors, that is, yellow (Y), magenta (M), cyan (C), and black (K) and develop the electrostatic latent image on the photosensitive medium into visible toner images. The toner image on the photosensitive medium is transferred to a transfer belt.
The developing units, the photosensitive medium, and the transfer belt are supplies that should be replaced when life spans thereof are completed. Thus these components are formed as detachable units. FIG. 1 shows an example of a conventional photosensitive medium unit.
Referring to FIG. 1, the conventional photosensitive unit 1 includes a photosensitive medium 2 having an outer circumference on which a photoconductive material is evaporation-coated. A frame 10 rotatably supports a rotary shaft 3 of the photosensitive medium 2. A handle 7 is connected to the frame 10, and a charging roller 5 evenly charges the outer circumference of the photosensitive medium 2. The electrostatic latent image is formed on the outer circumference of the photosensitive medium 2 during the printing process. The toner, that is, the developer, is supplied from the developing unit (not shown) to the electrostatic latent image to form a visible toner image. The visible toner image is transferred onto a transfer medium (not shown). Therefore, a portion 2a of the outer circumference of the photosensitive medium 2 facing the developing unit and a portion 2b facing the transfer medium are not covered by the frame 10, and, thusly, are exposed.
Since the outer circumference of the photosensitive medium 2 is exposed as described above, an optical fatigue phenomenon happens earlier. That is, sensitivity of the photoconductive material layer on the outer circumference of the photosensitive medium 2 with respect to the light is reduced. The optical fatigue phenomenon becomes worse when the photosensitive unit 1 is disposed out of the image forming apparatus, thereby resulting in a vague image being printed on the paper. In addition, if the optical fatigue phenomenon happens on the photosensitive unit 1, it may be decided that the life span of the photosensitive unit 1 is completed. Accordingly, the life span of the photosensitive unit 1 is reduced and costs for maintaining the supplies also increases since the photosensitive unit 1 should be replaced.